Urethanes for the treatment of rubber

ABSTRACT

NOVEL URETHANES HAVING THE FORMULA   C$NH.CO.O.N=Q=Z)M   WHERE X IS AN ORGANIC GROUP OF FUNCITONALITY M, Q IS AN AROMATIC GROUP IN THE QUINONOID CONFIGURATION, X IS AN IMINO GROUPS, AND M IS AN INTEGER EQUAL TO 1 OR MORE, MAY BE MADE BY REACTING AN ORGANIC ISOCYANATE WITH A NITROSOANILINE. THE COMPOUNDS ARE USEFUL IN THE TREATMENT OF RUBBER.

"United States Patent Office v 3,799,954 Patented Mar. 26, 19743,799,954 URETHANES FOR THE TREATMENT OF RUBBER Maurice Edward Cain,Welvwn Garden City, Geoffrey Thomas Knight, Shelford, and KeithFrederick Gazeley and Peter McHugh Lewis, Hitchin, England, assignors toThe Natural Rubber Producers Research Association, London, England NoDrawing. Original application Nov. 16, 1970, Ser. No. 90,138, now PatentNo. 3,721,659. Divided and this application Sept. 7, 1972, Ser. No.287,027 Claims priority, application Great Britain, Nov. 28, 1969,58,403/ 69 Int. Cl. C07c 125/06 U.S. Cl. 260-396 N 2 Claims ABSTRACT OFTHE DISCLOSURE Novel urethanes having the formula.

where X is an organic group of functionality m, Q is an aromatic groupin the quinonoid configuration, Z is an imino group, and m is an integerequal to 1 or more, may be made by reacting an organic isocyanate with anitrosoaniline. The compounds are useful in the treatment of rubber.

This application is divided out of our copending application No. 90138filed Nov. 16, 1970, now U.S. Pat. No. 3,721,659.

Our earlier copending application No. 752,174 filed Aug. 13, 1968, nowabandoned, relates to a process for improving the resistance todegradation of unsaturated natural and synthetic rubbers and to theimproved rubbery polymers prepared by the process. More particularly,our copending application is concerned with the formation of arubber-bound antioxidant which is formed by the chemical reaction of anaromatic nitroso compound with the molecules of the rubbery polymer.

Our said copending application makes use of aromatic nitrosophenols andnitrosoanilines in which the nitroso group is directly attached to anaromatic ring, preferably in the para-position to the hydroxyl or aminegroup. Such compounds are not very pleasant to handle, and objections ofpossible toxicity and derm'atitic activity have been raised againstthem. Also, they significantly reduce the scorch time of, and maypeptise the rubber in, rubber mixes into which they are introduced. Thepresent invention is an improvement in or modification of the basicinvention of application No. 752,174 in which the necessity of handlingthe aromatic nitroso compounds as such in order to admix them with therubber is avoided, and in which peptisation of the rubber and reductionin scorch time of the rubber mix is lessened.

The present invention provides compounds which are useful for improvingthe resistance to degradation of an unsaturated natural or syntheticrubber by providing antioxidant groups bound to the rubber molecules.

The treatment of rubber using these compounds is the subject of our U.S.patent application Ser. No. 9013-8 referred to above.

The compounds are useful for the treatment of both natural and syntheticpolymers containing unsaturated carbon-carbon linkages, or other groupscapable of suitable reaction with the aromatic nitroso compounds definedabove, but are not suitable for use with rubbers normally regarded assaturated polymers which contain very low amounts of unsaturation forvulcanization purposes, for example, ethylene-propylene terpolymers andbutyl rubber. Examples of suitable unsaturated synthetic rubbers arecis-polyisoprene, polybutadiene, styrene-butadiene copolymers,acrylonitrile-butadiene copolymers and polychloroprene. The treatment isapplicable to unsaturated natural or synthetic rubbers both in the latexand in the dry form.

The present invention provides a urethane having the general formula Xis an organic group of functionality m,

Q is an aromatic group in the quinonoid configuration, Z is an iminogroup, and

m is an integer equal to 1 or more.

The nature of the group X is not critical to this invention. The group Xmay be regarded as the nonfunctional residue of the isocyanate startingmaterial. The group X will generally be inert, and will often be analiphatic, aromatic or aliphatic-aromatic group (of functionality m)containing carbon and hydrogen only.

It will be understood that the value of mis equal to the functionalityof the isocyanate used as a starting material.

Q is a phenylene or substituted phenylene group in the quinonoidconfiguration, preferably an unsubstituted p-phenylene group in thequinonoid configuration.

A convenient way of making these novel compounds is by heating theappropriate mono-C-nitrosoaniline with the appropriate organicisocyanate.

The aromatic nitroso compound which is one component of the urethane ispreferably one in which the nitroso group is joined to the arylene groupin the para-position to the 'NHR group. The arylene group may be aphenylene or a substituted phenylene group, provided always that thenitroso compound is capable of forming a urethane with an organicisocyanate. The group R may be a C to C alkyl group or a phenyl group,or may be any other substituent, provided again that the nitrosocompound is capable of forming a urethane with an organic isocyanate.Examples of suitable aromatic nitroso compounds are4-nitrosodiphenylamine and N-hexyl 4- nitrosoaniline.

The nature of the organic isocyanate, which is the second component ofthe urethane, is not critical. It may be a mono-isocyanate, for examplephenyl isocyanate, or a dior polyisocyanate, for exampletoluene-2,4-di-isocyanate, 4,4'-di-isocyanatodicyclohexylmethane, or4,4- di-isocyanatodiphenylmethane.

The urethane may be prepared by conventional methods by warming thearomatic nitroso compound with the organic isocyanate, optionally in thepresence of an inert solvent, for example to a temperature of from 50 C.to C. for from 10 to 60 minutes.

Preferably substantially equivalent amounts of the two reactants areused, e.g. so as to provide from 0.8 to 1.2 isocyanate groups forreaction with each aromatic nitroso molecule. The course which isbelieved to be followed during the reaction may be represented asfollows:

Reaction (a) in the scheme is the reversible formation of an oxime fromthe nitroso compound. The equilibrium of this reversible reaction, whichis heavily on the left hand side of the equation under normalcircumstances, is displaced to the right by removal of the oxime as itis formed through reaction (b). Reaction (b) takes place on warming thereactants together. In fact, reaction (b) is also reversible, since theurethanes decompose on being heated to their melting points to give thearomatic nitroso compound and free isocyanate; it is on thisdecomposition that the rubber treatment process of this inventiondepends.

Reaction (b) goes only with some difficulty and it may be necessary touse a relatively reactive organic isocyanate. We have found thataromatic isocyanates, and particularly aromatic di-isocyanates are morereactive in this reaction than aliphatic isocyanates. The use ofcatalysts may also be advantageous.

It will be noted that the reaction scheme requires the presence of atleast one hydrogen atom attached to the amine nitrogen atom. We havefound that the reaction does not work if unsubstituted nitrosoanilinesare used, because substituted ureas are formed in preference tourethanes.

The compounds described above are not true urethanes, being the reactionproducts of isocyanate with =NOH, rather than with -COH. Their fullchemical names are, however, complex, and they are referred to, forconvenience, in this specification and claims as urethanes.

The amount of the urethane used in the treatment of rubber is preferablysuch as to provide from 0.2% to 3.0% by weight of the aromatic nitrosocompound based on the weight of the rubbery polymer, more preferably0.2% to 2.0% for latex compositions and 0.5% to 3.0% for dry rubbercompositions.

In the case of latex compositions, the mixture of the rubber with theurethane may be formed by mixing solutions or ball-milled dispersions ofthe urethane prereaction product with the rubber latex, followed asdesired by air-drying, coagulation, dipping or foaming by well knowntechniques.

In the case of dry rubber mixes, the urethane may be added to the rubberduring normal compounding, care being taken that the temperature of themixture does not rise to such an extent as to cause prematuredecomposition of the urethane.

It is believed that the urethane reversibly decomposes at an elevatedtemperature to give the aromatic nitroso compound which then reacts withthe rubber, and an organic isocyanate. The temperature of decompositionof the urethane depends on its structure, but we have found that thedesired reaction can generally be achieved by heating a dry rubbermixture at a temperature of from 100 C. to 250 C., preferably from 140C. to 180 C., for from 1 to 60 minutes, higher temperatures requiringshorter heating times. It is believed that the aromatic nitroso compoundreacts with the rubber substantially as fast as it is liberated. Forlatex, lower temperatures are generally appropriate, for example, from60 C. to 130 C., preferably 90 C. to 120 C. It may even be possible toobtain the desired reaction by leaving the latex mixture at ambienttemperatures for some days. It may be that the alkaline hydrolysis ofthe urethane in the latex enables the desired reaction to take place atlower temperatures than are possible with dry rubber compositions.

The urethanes can be used to protect raw or vulcanized rubber fromoxidative degradation. Where the rubber is to be vulcanized, it isconvenient to effect vulcanization using the same heating step. To thisend, the heating step of the process of the present invention may beperformed in the presence of such other fillers, additives, vulcanizingagents or other compounding ingredients as may be required for the finalrubber product. While the urethanes of this invention can be used toprotect unsaturated rubbers generally from oxidative degradation, it ispreferable to select a urethane which decomposes in a temperature rangeappropriate to the rubber system to be protected.

It is believed that the reaction efficiency may be higher when aurethane is used than when the free nitroso compound is added direct tothe rubber mix. Heating of the urethane-rubber mixture results inrelease of the nitroso compound which disappears, by reaction with therubber, as fast as it is formed. It is thought that the presence, at anymoment, of substantial quantities of free nitroso compound may reducethe reaction efliciency by initiating unwanted side reactions withintermediates.

If the urethane decomposes at too low a temperature, prematuredecomposition may take place, resulting in the presence in the rubbermix of substantial quantities of free nitroso compounds, which ma reducereaction efficiency and scorch time. If the urethane decomposes at toohigh a temperature, lengthy heating of the rubber mix may result inover-vulcanization.

The only really satisfactory way of determining the decompositiontemperature of a particular urethane under conditions where the nitrosocompound is removed as it is formed, is to test the urethane in a rubbermix. It does, however, appear that the decomposition temperatures ofthese urethanes are roughly proportional to their melting points. Itfurther appears that the melting points of urethanes formed by reactingthe same nitroso compound with different isocyanates, depend on thereactivity of the isocyanate, being higher for more active (e.g.aromatic) isocyanates.

In the examples, which are included to illustrate the various aspects ofthe invention, the following abbreviations have been used:

Nitroso compounds:

NDPA:4-nitrosodiphenylamine HNA: N-hexyl-4-nitrosoaniline IsocyanateMDI=4,4'-di-isocyanatodiphenylmethane Parts and percentages are byweight throughout, and temperatures are in C.

The general method of preparation is as follows:

The mono-isocyanate or di-isocyanate is dissolved in sodium dry toluene(250 ml.) in a 500 ml. flask fitted with magnetic stirrer, nitrogeninlet and a reflux condenser protected by a calcium chloride dryingtube. The solvent is flushed with nitrogen before addition of theisocyanate, the nitrogen inlet being well below the surface of theliquid, and a gentle nitrogen stream is maintained through theapparatus. The nitrosoamine is added and the mixture heated withstirring until solution is complete.

EXAMPLE 1 Phenyl isocyanate (25 mmoles) was dissolved in sodium drytoluene as above, and NDPA (23 mmoles) added. The mixture was heated toreflux in the toluene for 1 hour, the bath turned ofl and the flaskallowed to cool overnight to room temperature. The black tarry solid wasremoved by filtration, and the filtrate cooled to 70 C., when a brownsolid separated. This was removed by filtration, washed with coldtoluene, and dried under vacuum. The yield 15%, M.P. -110 (dec.).

EXAMPLE 2 NDPA (0.1 mole) and MDI (0.05 mole) were dissolved in sodiumdry toluene (250 ml.) under nitrogen as above and the solution heated inan oil bath at 100- C. for one hour. The flask was allowed to cool toroom temperature, the brown solid removed by filtration, washed withtoluene and dried under vacuum. It had M.P. 160162 (dec.), the yield was80%.

EXAMPLE 3 A solution of N-hexyl-4-nitrosoaniline (50 mmoles) and MDI (25mmoles) in sodium dry toluene ml.) was prepared as above andtriethylenediamine (4 mole percent on HNA) added as catalyst. Thesolution was heated in an oil bath at 60 C. for 25 minutes, allowed tocool to room temperature and the green precipitate removed byfiltration, washed with toluene and dried under vacuum. The yield was80%, M.P. 124 (dec.).

Identification of the derivatives The absence of isocyanate groups wasconfirmed by the absence of the NCO band at 2260 emsin the infraredspectra. The urethane carbonyl band was present 5 6 in all cases at1720-17 80 cm.- whereas the correspond- 2. A urethane as claimed inclaim 1, wherein Q is a ing urea derivatives of the nitrosoanilines(reaction at p-phenylene group in the quinonoid configuration. the NHgroup) would have a typical band at 1660 cm.- and this was absent.References Cited 2 i h th 1 f I 5 UNITED STATES PATENTS we 6 genera a3,352,750 11/1967 Buntin 260-396N h t =Q= )m 3,645,980 2/1972 Baker etal 260-396 N w ere X is a hydrocarbon group, VIVIAN GARNER, PrimaryExaminer Q is a phenylene group in quinonoid configuration, 10 Z is animino group of formula NR wherein R is alkyl U.S. C1. X.R. Of 1-6 carbonatoms 01' phenyl, and 5 1, 92 3,

m is an integer equal to 1 or 2.

